Quark
In 1964, two physicists named Murray Gell-Mann and George Zweig independently proposed a radical idea. They suggested that the chaotic collection of subatomic particles known as hadrons were not fundamental. Instead, these particles were made of smaller components they called quarks. The physics community reacted with skepticism. Many scientists viewed quarks as mere mathematical abstractions rather than physical realities. This doubt persisted for several years until experimental evidence began to emerge.
The first concrete proof arrived in 1968 at the Stanford Linear Accelerator Center. Deep inelastic scattering experiments revealed that protons contained point-like objects inside them. Physicists Richard Taylor, Henry Kendall, and Jerome Friedman observed these collisions and published their findings on the 20th of October 1969. They initially called these internal structures partons because they were unsure if they matched the theoretical quarks. It took time for the scientific consensus to shift from abstract math to tangible matter.
Quarks exist in six distinct varieties known as flavors. These include up, down, charm, strange, top, and bottom. Up and down quarks possess the lowest masses of all types. They form the stable matter found throughout the universe today. Heavier quarks like charm, strange, bottom, and top decay rapidly into lighter versions through particle interactions.
These six flavors group themselves into three generations. The first generation contains up and down quarks. The second generation includes strange and charm quarks. The third generation consists of bottom and top quarks. Each higher generation carries greater mass and less stability than the one before it. No evidence exists for a fourth generation of quarks or other elementary fermions. Studies of the Z boson resonance width suggest any additional neutrino would be too heavy to exist within current physical models.
The discovery timeline spans decades of intense experimentation. Strange quark existence was indirectly validated by scattering experiments at SLAC. This validation explained the behavior of kaons and pions discovered in cosmic rays back in 1947. A theoretical model called the GIM mechanism proposed by Sheldon Glashow, John Iliopoulos, and Luciano Maiani required an undiscovered charm quark to function correctly.
Charm quarks were produced almost simultaneously in November 1974. One team led by Burton Richter worked at SLAC while another under Samuel Ting operated at Brookhaven National Laboratory. They observed mesons containing bound charm quarks. These particles became known as J/psi mesons after receiving different symbols from each group. The bottom quark appeared in 1977 through Fermilab research led by Leon Lederman. Finally, the top quark emerged in 1995 via CDF and DØ teams at Fermilab. It possessed a mass nearly equal to that of a gold atom.
Quarks carry a property called color charge which comes in three types: red, green, and blue. Despite the name, this has no relation to visible light colors. Every quark holds one color value while every antiquark carries an anticolor. The strong interaction binds these particles together through force carriers known as gluons. Quantum chromodynamics describes how this attraction works mathematically.
Gluons themselves carry color charge allowing them to emit and absorb other gluons. This creates a phenomenon called asymptotic freedom where binding forces weaken as quarks approach each other. Conversely, pulling quarks apart strengthens the bond like stretching an elastic band. Eventually new pairs form before separation occurs. This process ensures quarks never appear in isolation within normal conditions. Only the top quark may decay before forming hadrons due to its extreme instability.
Under sufficiently high temperatures around 2 trillion kelvin, quarks can become deconfined from bound states. They propagate freely as thermalized excitations within a larger medium. Scientists call this state quark-gluon plasma. It likely filled the universe during the first fractions of a second after the Big Bang. Experiments at CERN in the 1980s and 1990s attempted to recreate this condition without full success.
Recent work at the Relativistic Heavy Ion Collider shows evidence for liquid-like quark matter. This substance exhibits nearly perfect fluid motion despite being composed of subatomic particles. Another theoretical phase exists under high baryon densities and low temperatures similar to neutron stars. In this environment quark Cooper pairs condense creating color superconductivity. Color charge passes through such matter with no resistance breaking local symmetry groups.
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Common questions
Who proposed the existence of quarks in 1964?
Murray Gell-Mann and George Zweig independently proposed the existence of quarks in 1964. They suggested that hadrons were made of smaller components called quarks.
When was the first concrete proof of quarks discovered at Stanford Linear Accelerator Center?
The first concrete proof arrived in 1968 at the Stanford Linear Accelerator Center through deep inelastic scattering experiments. Physicists Richard Taylor, Henry Kendall, and Jerome Friedman published their findings on the 20th of October 1969.
What are the six distinct varieties or flavors of quarks?
Quarks exist in six distinct varieties known as flavors including up, down, charm, strange, top, and bottom. Up and down quarks possess the lowest masses while heavier quarks decay rapidly into lighter versions.
How many generations of quarks exist according to current physical models?
These six flavors group themselves into three generations with no evidence for a fourth generation of quarks or other elementary fermions. The first generation contains up and down quarks, the second includes strange and charm quarks, and the third consists of bottom and top quarks.
When did scientists discover the charm quark and which teams led the research?
Charm quarks were produced almost simultaneously in November 1974 by two separate teams. One team led by Burton Richter worked at SLAC while another under Samuel Ting operated at Brookhaven National Laboratory.
Under what conditions can quarks become deconfined from bound states?
Under sufficiently high temperatures around 2 trillion kelvin, quarks can become deconfined from bound states. Scientists call this state quark-gluon plasma and it likely filled the universe during the first fractions of a second after the Big Bang.